This invention relates generally to apparatus for digesting organic material to produce a fuel gas or biogas. More specifically, the invention is directed to a type of digester which has an elongated reaction chamber with an inlet opening, an outlet opening, and means for collecting the gas which is produced in the reaction chamber. Flowable organic material is fed into the reaction chamber through the inlet opening, flows through the chamber toward the outlet opening, and passes out of the chamber through the outlet opening. As the organic material flows through the reaction chamber between the inlet and outlet openings, it is digested by anaerobic or aerobic action to produce a fuel gas or biogas, such as methane mixed with CO.sub.2.
Digesters of the type described above are generally constructed of concrete, since this material is relatively inexpensive, non-corrosive and long-lasting. The digester is constructed by assembling the digester at the site from precast concrete sections. It is also common practice to heat the digester to accelerate the microbial digestion.
One of the problems encountered in use of concrete for the construction of digesters is, that this material has very poor thermal insulating value. Therefore, much of the fuel which is collected from the digestion process is used for maintaining the temperature of the digester. This seriously effects the efficiency of the digester.
Attempts have been made in the past to insulate the digester by lining the inside or the outside surface of the digester. In either case, the insulation must be frequently replaced since those materials which have good thermal insulating qualities do not usually have good wear qualities and the wearing process further reduces the R-valve of the insulation. Another problem frequently encountered in the operation of digester systems is that the organic material has a tendency to form a crust at the top surface. This crust prevents gas which is generated in the organic material from passing out of the material to be collected. Also, the crust reduces the active area for digestion to take place. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a digester for producing fuel gas from organic material, which digester has a higher operating efficiency than the known prior art digesters.
Another object of this invention is the provision of a digester having a construction which combines strength and durability with high thermal insulation.
A further object of the present invention is the provision of a digester in which the organic material is constantly agitated to prevent crusting at the upper surface of an organic material.
It is another object of the instant invention to provide a digester in which the organic material is agitated by means of some of the gas which is generated from the material.
A still further object of the invention is the provision of a digester in which the organic material is heated, while the flow of organic material is resisted by means of a baffle to enable the material in the upstream side of the baffle to be properly heated.
A further object of the invention is to provide a digester in which a small space is created between the upper level of the organic material and the top of the digestion chamber to receive gas which is generated from the organic material.
It is a further object of the invention to provide a digester which includes a collection chamber located above the digester chamber and which includes means for conveying generated gas from the digestion chamber to the collection chamber.
It is a further object of the invention to provide a digester which is simple in construction, which is inexpensive to manufacture, and which is capable of a long life of useful service with a minimum of maintenance.
It is a further object of the invention to provide a digester which can be cleaned periodically with a minimum of effort.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.